x <- 1:10
n <- length(x)

idxs <- seq(from = 1, to = n, by = 2)
s1 <- mean(x[idxs])                     # 25
s2 <- mean2(x, idxs = idxs)             # 25
stopifnot(identical(s1, s2))

idxs <- seq(from = n, to = 1, by = -2)
s1 <- mean(x[idxs])                     # 25
s2 <- mean2(x, idxs = idxs)             # 25
stopifnot(identical(s1, s2))

s1 <- mean(x)                           # 55
s2 <- mean2(x)                          # 55
stopifnot(identical(s1, s2))
